Phenothiazine-based donor–acceptor covalent–organic frameworks with keto–enol irreversible tautomerism as a promising third-order nonlinear optics material

Abstract

Covalent–organic frameworks (COFs) exhibit high structural designability and functional tunability, and have become a promising platform for designing excellent third-order nonlinear optical (NLO) candidate materials. However, their third-order NLO performance is limited by insufficient intramolecular charge transfer. Herein, we synthesized two phenothiazine-based donor–acceptor (D–A) COF films on transparent glass panes via a Schiff base condensation reaction using a solvothermal method. The TFP-TZ using triformylphloroglucinol as an acceptor linker and a phenothiazine-based donor unit exhibits a fully β-ketoenamine conformation due to the irreversible keto–enol tautomerization. At a pulse energy of 15 μJ, TFP-TZ films exhibited nonlinear saturated absorption, with a nonlinear absorption coefficient (β) of −6.78 × 10⁻⁷ m W⁻¹, a nonlinear refraction coefficient (n₂) of −1.67 × 10⁻¹³ m² W⁻¹, and a third-order nonlinear polarization coefficient (χ⁽³⁾) of 4.73 × 10⁻⁷ esu. These values are 6.65, 1.82, and 1.91 times higher than those of TF-TZ films imine-linked without the carbonyl group, respectively. The combination of D–A charge transfer properties and irreversible interconversion of salicylideneaniline molecular units results in narrowing of the band gap of COFs, thereby enhancing their third-order NLO performance.